Subaqueous basaltic magmatic explosions trigger phreatomagmatism: a case study from Askja, Iceland

38 Sequences of basaltic pillow lavas that transition upwards with systematic gradation from pillow 39 fragment breccias to fluidal bomb-bearing breccia to bomb-bearing lapilli tuffs are common at 40 Askja volcano, Iceland. Based on the detailed textural investigation of three of these sequences, 41 we argue that they record temporally continuous transition from effusive to explosive products 42 that were erupted from and deposited at or near a single subaqueous vent. The recognition of 43 such sequences is important as they provide evidence for controls on the onset of explosive 44 activity in subaqueous environments. Such investigations are complicated by the interplay of 45 magmatic gas expansion, phreatomagmatic and mechanical granulation fragmentation 46 mechanisms in the subaqueous eruptive environment. 47 All of the sequences studied at Askja have textural, componentry and sedimentological 48 characteristics suggestive of a close genetic and spatial relationship between the pillow lavas and 49 all of the overlying glassy clastic deposits. The identification of magma fragmentation signatures 50 in pyroclasts was accomplished through detailed textural studies of pyroclasts within the full 51 range of grain sizes of a given deposit i.e. bomb/blocks, lapilli and fine ash. These textural 52 characteristics were compared and evaluated as discriminators of fragmentation in pyroclastic 53 deposits. The presence of angular vitric clasts within the breccia and lapilli tuff displaying fragile 54 glassy projections indicates little or no post-depositional textural modification. A shift in vesicle 55 and clast textures between the pillow lavas and the large concentration of fluidal bombs in the 56 breccia indicate that the phreatomagmatic explosions were initially triggered by magmatic 57 vesiculation. The initial magmatic gas expansion may have been triggered by depressurization 58 caused by the drainage of the ice-confined lake surrounding Askja. The Fuel Coolant Interactions 59 3 (FCI) of the more efficient phreatomagmatic explosion was enabled by the increase in the 60 surface area to volume ratio of the fluidal bombs in the water, producing a premix of magma and 61 water. The onset and increasing influence of phreatomagmatic fragmentation is preserved in the 62 presence of very fine blocky ash particles and diminished presence of larger particles such as 63 fluidal bombs. The textural, sedimentological and environmental characteristics of these deposits 64 suggest that phreatomagmatic explosions can be triggered by initial magmatic gas expansion, but 65 that it is likely one of many mechanisms for triggering such explosions. 66 67